This invention relates to absorbance monitors or detectors and more particularly relates to gas discharge lamp control circuits for absorbance monitors.
Gas discharge lamps are difficult to operate efficiently because their starting voltages are much higher than their running voltage. Typical values are 2000 volts starting and 180 volts running.
In one type of lamp control circuit intended to address this problem, high potential peaks are applied across the lamps to cause them to ignite and then a lower potential AC is applied to the lamp to keep it illuminated. After warm-up of the lamp, oscillations caused by different ionization paths within the lamp are reduced by narrow blanking pulses.
In a prior art type of absorbance monitor of this class, separate circuits or changes in material circuit parameters are used to apply the high voltage starting pulses and the lower voltage operating potential, and frequency is set and controlled at the set frequency or maintained at a constant rate.
This type of prior art absorbance monitor has several disadvantages such as, for example: (1) it frequently requires an expensive transformer; (2) there is excessive baseline noise; (3) the blanking pulses sometimes prevent ignition during start-up and warm-up; (4) it is energy inefficient, heavy or bulky; and (5) its lamp life is relatively short.
In another type of prior art circuit, a "flyback transformer" is used to operate high voltage pulses. Its disadvantages are poor form factor of the current through the lamp, an enhanced tendency to cause the gas discharge lamp to rectify, thus reducing the life due to deleterious electrode effects; and comparatively large size and weight of the transformer due to inefficient use of the magnetic energy storage capabilities of its core.
In yet another type of prior art circuit, a transformer is used whose core saturates at the end of each cycle or half cycle when the lamp is running as well as when starting. This type of prior art circuit has a disadvantage in that the saturation during running wastes power and is energy inefficient.